The present invention relates generally to methods of making intercell welds in electric storage batteries and in particular to those methods where electrodes are oriented on either side of a partition or other aperture in a battery case, lugs or other battery parts are placed over each side of that aperture, contact produced therebetween, current passed through the lead parts to melt the same and pressure applied generally for the purpose of providing an electrical connection through the aperture. Such an apparatus is disclosed in U.S. Pat. No. 4,013,864.
It is long known that lead parts on either side of a partition with an aperture formed therein can be welded through that aperture using anyone of a number of techniques. In the lead acid battery art, the most common site for welding through an aperture is during the formation of intercell connections, that is, in making the electrical connections between one battery cell and the next and/or between the end most battery cells and the exterior battery terminal of the battery. Since connections through apertures of this sort, in addition to being electrical, must provide a liquid seal from cell to cell to prevent "pumping", some attention has been directed in the concerning various methods for insuring that an intercell connector, in addition to providing a good electrical connection, will also exhibit good sealing characteristics.
U.S. Pat. No. 3,687,734, generally discloses a connector for electrically connecting two elements of a storage battery through an aperture wherein at least one of the connectors lugs is provided with a passage extending therethrough. Molten material from an external portion of the connector exists through this passage during the heat fusion step as a result of pressure from a heat energy build up. The patentee attempts by this structure to avoid the problem of blow outs or lead expulsion which has been encountered by many practitioners in this art.
Another attempt to avoid "blow out" is disclosed in U.S. Pat. No. 3,476,611, wherein an intercell connections are by a projection welding process from pins which extend through the partition between two adjacent battery compartments, which pins have dimensions such that, when they are fluidized during the process, the volume of the pin material does not exceed the volume defined by the walls of the opening.
Such attempts inherently involve tolerance and positioning problems during the manufacturing and assembly of the parts in battery. More recently, other methods have been developed for producing battery intercell electrical connections, which methods have generally been referred to as "extrusion-fusion" type method. For example, in U.S. Pat. No. 3,793,086, a method is disclosed wherein flat surface connector lugs are placed on each side of the battery partition wall, adjacent an aperture. The connector lugs are extruded by a pair of opposed electrodes into the aperture until they meet, whereupon an electrical welding current is applied. When the welding current ceases, the connectors are allowed to cool. As stated in this patent, "it should be emphasized that reduction of the initially applied shear force during the welding cycle is essential. If the high applied shear force is maintained during the welding cycle, molten lead will be squirted from the welding joint and an imperfect joint can result."
Another approach to the problem of "blow outs" is that disclosed, for example, in U.S. Pat. No. 4,046,062, wherein separate hold down sleeves are employed to clamp the lugs into sealing engagement with the partition wall aperture prior to and during the extrusion-fusion process. After clamping, metal is extruded until contact is made, electric current is then passed through the extruded metal to melt it, and, under the continuing force of the electrodes, metal is caused to flow into any voids in the aperture while, at the same time, extruding more metal out of the lugs into the aperture until the aperture is packed full of lug metal.
In U.S. Pat. No. 3,896,316, a similar extrusion-fusion system is disclosed wherein high density polyurethane pads are provided around each of the electrodes to clamp the lugs tightly against the walls of the intercell connection during the extrusion, fusing the cooling steps.
In U.S. Pat. No. 3,723,699, the problem of blow outs or lead expulsion is expressed in a projection welding context wherein the lugs are provided with upstanding annular ribs which surround the hole in the partition wall and are caused to bite into the partition wall, in an attempt to minimize flow and to provide a good mechanical key between the lugs and partition wall, thus minimizing the possibility of relative sliding movement between the lugs and partition wall.
Accordingly, as seen from the above described prior art references, considerable problems have been experienced with blow outs, particularly where lead is extruded into the aperture of the partition with the intent of filling the same. Extrusion-fusion welding processes have none the less achieved considerable success in the industry.
Since the development of extrusion-fusion welding processes, other problems have also been encountered in controlling the types of welds which are obtained under actual production conditions in battery plants. It is long been known, for example, that variations in such parameters as the electrical welding current, squeezing pressure, and in the dimensions of the connector lugs and/or the partition or casing wall thickness has a direct effect on the quality of the weld. In the past, these parameters were adjusted and the resulting welds were then inspected to determine the quality thereof. This adjustment and inspection process continued until welds meeting the predetermined criteria were obtained.
After this initial set up procedure, which can be very time consuming and laborious, an attempt is then made to maintain these parameters constant throughout the subsequent production. Unfortunately, changes in one or more of these parameters during the ensuing production could have a detrimental effect on the quality of the welds produced thereby. In some cases, these welds will tend to over heat and blow out, while in other cases, cold or incomplete welds may be formed.
Although, as stated above, a number of conditions may account for the variability from weld to weld, I have found that the condition of the lugs contribute substantially to the great variability in weld performance. Lugs are typically cast of lead alloys which harden as they age, and, which during the preceding battery manufacturing operations may acquire varying surface characteristics. For example, lugs which are cast in conventional parts molding machines may have a slight residue of oil or other film which adheres thereto from the casting machines (as, for example, films used to aid in the release of the parts which are cast).
Lugs may also age in the battery plants for varying lengths of time depending upon their sequence of use. During storage they may be exposed to varying quantities of lead oxide dust and/or other contaminents present in the battery plant, and may be subjected to various additional contaminents depending upon the operations employed to fuse those lugs to their respected straps. In some battery manufacturing plants, the lugs may be cast in a "cast-on" machine or may otherwise be formed and fused to the straps with a much lesser degree of aging prior to the final battery assembly.
Once associated with the groups, the lugs are particularly prone to contamination as a result of shedding or other direct or indirect exposure to the active material of there associated groups or elements. Finally, depending upon the manufacturing process employed to make the cases, variable contaminations of the lugs surfaces routinely occurs.
Prior art extrusion-fusion techniques, to the extent they have attempted to control parameters of surface contamination, tend to rely on sensing the establishment of an electrical current path through the lugs at or near the completion of the extrusion process, that is, at the time contact is created between the lugs within cellular aperture. Unfortunately, variations in surface contaminations of the lugs considerably change the surface resistance of the lugs; therefore, methods in which the establishment of a certain degree of electrical contact within the aperture is sensed, tend to begin the weld cycle relatively earlier or later depending upon how dirty the lug surfaces are at their points of contact, i.e., how much lug contact area need be established in order to trigger the weld cycle.
Other prior art methods have focused upon controlling the pressure of extrusion to thereby presumably establish a uniform contact area between the lugs, regardless of there surface contamination. Due to the variability aging processes however, and the consequent variations in the hardness of the lugs to be extruded (depending upon the age of those lugs), close control of the extrusion pressure from weld to weld results in variations of contact area between the lugs.
Other prior art methods have used mechanical stops to limit the extrusion of the extruding apparatus in order to establish a uniform contact area between the lugs. However, variations in lug dimensions will effect the contact area thereby rendering this technique less than satisfactory.